Cryogenic air separation processes separate pressurized air feed streams into individual product streams enriched in oxygen, nitrogen, and in some cases, argon. The cryogenic process is based on cooling the pressurized air feed streams to near or below their dew points, followed by separation in one or more distillation columns. A typical process involves separating a portion of nitrogen from the air feed streams in a first, higher pressure distillation column, followed by separation of oxygen from the remaining feed streams in a second, lower pressure distillation column. The higher pressure distillation column is normally operated slightly below the air feed pressure. The lower pressure distillation column operates at a pressure that allows liquid oxygen in the sump to be boiled against condensing, pressurized nitrogen from the overheads of the higher pressure distillation column, or condensing, pressurized air feed streams. The pressure ratio between the higher and lower pressure distillation columns is normally in the range of 2.5 to 5.0 to 1. The majority of cryogenic air separation plants currently in operation have lower pressure distillation columns operating at less than 10 psig, and higher pressure distillation columns operating at between 60 to 100 psig depending on the purity of the resulting product streams and specific equipment design parameters internal to the process.
In order to be distributed and usefully employed in downstream processes, the product streams are normally pressurized to levels well in excess of the operating pressure of the corresponding distillation column from which they are produced. A traditional method to accomplish product stream pressurization is to withdraw the product stream from the cryogenic process in the gaseous phase and compress it to the desired pressure. Compression equipment can utilize one or more stages, with or without cooling of the gas stream between stages of compression. Reciprocating, screw, centrifugal and axial compression equipment have been used to compress air separation process feed and product streams. Another method that has been employed to pressurize product streams to levels slightly elevated above the pressure of the distillation column from which they are produced involves the use of liquid head pressure. If a distillation column is elevated in relationship to heat exchange equipment to which it is connected, a liquid stream removed from the distillation column will be at a higher pressure at the connection to the inlet of the heat exchanger. The weight of the column of liquid between the heat exchanger and distillation column causes the increase in pressure at the inlet to the heat exchanger. The liquid product stream is then vaporized and warmed in the heat exchanger and delivered as a gaseous product stream at a pressure normally 1 to 10 psi higher in pressure than the pressure of corresponding distillation column from which it was produced.
Another means of increasing the pressure of a product stream from a cryogenic air separation process is by removing a liquid stream from the distillation columns, pumping the stream to higher pressure, and vaporizing and warming the pumped liquid stream in heat exchange equipment. These methods are typically described as pumped liquid or internal compression processes. The product streams can be any of the enriched streams produced as liquid within the cryogenic process and can be delivered at the desired pressure or further compressed to higher pressures before delivery. A common feature of these processes is the provision of fluids that can be heat exchanged against the pumped product streams in order to recover and return refrigeration back to the distillation system. The fluids used in the heat exchange process are often air or nitrogen streams that are often provided at pressures higher than the operating pressure of the main feed air streams or distillation column operating pressures from which they enter or from which they are produced. The fluids are normally higher in pressure than the pumped product streams they are heat exchanged against, but may be equal to or lower than the pressure of the pumped product streams, particularly when the product streams are near or above their critical points.
Pumps for pressurizing the liquid product streams may be of horizontal or vertical design and are typically driven with electric motors. Several pumps for the same service are often interconnected to allow for redundancy in the case of failure of one or more units. Because of the piping, valving, instrumentation and need to locate the motor drives at ambient environmental conditions, all of the foregoing equipment items are sometimes grouped together in a separate insulated enclosure, or pump box, separate from the distillation column systems. Liquid product lines to and from the pump box are interconnected to the distillation system and cryogenic heat exchange system via other insulated enclosures referred to as crossovers. Alternatively, and usually in the case of smaller production facilities, the liquid product pumps and their associated equipment may be contained in the bottom of the same insulated enclosure containing the distillation and/or heat exchange systems.
Pressurizing cryogenic liquids contained in storage tanks is normally accomplished by means of vaporizing a portion of the liquid inventory and admitting the resulting vapor into the tank's vapor head space. This method is thermodynamically inefficient for processes requiring liquid product, since a potion of the liquid is lost through vaporization. Another disadvantage occurs if the contents of the tank are to be quickly discharged or operated in a cyclic nature, due to the need for large vaporization equipment to quickly generate vapor to replace liquid inventory and maintain constant pressure.
U.S. Pat. No. 6,038,885 describes a pumped liquid process in which liquid product streams are removed from the distillation system, conveyed to an inventory accumulation tank, pumped to increase pressure, followed by vaporization and warming in the heat exchange system. In this example an air stream at the appropriate pressure is used to recover refrigeration from the pumped product streams. Also in this example, the use of at least two pumps are noted for pressurizing the product stream.
A mechanical pump design and its placement internally or externally to a cryogenic liquid storage vessel (liquefied natural gas) is disclosed in U.S. Pat. No. 5,884,488.
U.S. Pat. No. 5,666,823 describes a pumped liquid process in which an oxygen product stream and a separate nitrogen product stream are pumped to increase pressure prior to vaporization and warming. Higher pressure air and nitrogen streams, singly or in combination, are used to recover the refrigeration from the pumped product streams.
A triple distillation column, pumped liquid oxygen product process is disclosed in U.S. Pat. No. 5,341,646. In this process the pressure of the feed air is sufficient for use in recovering refrigeration from the pumped liquid oxygen stream.
U.S. Pat. No. 5,136,852 describes the prior art and an improvement for the pressure control of cryogenic liquid storage tanks. Both prior art and the invention require the input of heat into the system with subsequent loss of liquid inventory in order to control the pressure of the storage tank.
The non-mechanical pumping of a fluid from a vessel by generating gas pressure within the vessel is disclosed in U.S. Pat. No. 4,852,357. An electrical resistance heater is immersed in the vessel to vaporize liquid inventory to generate pressure within the gas head space of the vessel. The invention requires the input of heat into the system with subsequent loss of liquid inventory in order to control the pressure of the storage tank.
EP 0,949,473 A1 discloses the collection of liquid inventory within an air separation process for reintroduction to the column systems to shorten the time required to restart the air separation plant. Transfer of the liquids collected in a temporary holding tank to the distillation columns is accomplished by pressurizing the tank with a higher pressure gas. The liquids are pressurized to a level equal to the distillation column pressure plus liquid head pressure differences between the tank and column.
An air separation process incorporating mechanical pumps to provide varying amounts of pressurized oxygen and nitrogen products is described in WO 97/04279. Other representative pumped liquid air separation processes are described in U.S. Pat. Nos. 5,355,681; 5,901,576; 5,907,959; 5,956,973; 5,956,974; 5,966,967; and 6,009,723.
Pressurizing tanks containing liquid by introducing a higher pressure gas into the gas head space of a tank is known for transferring liquid on an intermittent flow basis. The use of gas pressurization for transferring liquid products on a continuous basis, however, combined with the efficient recovery of vented gas head space inventory into an air separation process, has not been recognized in the prior art as an efficient method to replace mechanical pumping systems. The invention disclosed below and defined by the claims which follow addresses the need for improved designs and methods of operation for pressurizing product streams internal to an air separation process.